Disk substrate and optical disk

ABSTRACT

A data area to record and/or reproduce data and an eccentricity measuring area  14  in which a groove area formed with spiral grooves and a planer mirror area are spatially alternately arranged are provided for a disc substrate. A width of groove area formed in the eccentricity measuring area, a width of mirror area, and an interval between the grooves in the groove area are selected so that a conventional mechanical characteristics measuring apparatus can track a plurality of grooves formed in the groove area as if the grooves were a single groove.

TECHNICAL FIELD

The invention relates to a disc substrate and an optical disc. Moreparticularly, the invention is suitable when it is applied to an opticaldisc in which an information signal portion and a light transmittinglayer are sequentially formed on a disc substrate and an informationsignal is recorded and/or reproduced by irradiating a laser beam fromthe side where the light transmitting layer is formed.

BACKGROUND ART

In recent years, it is demanded to further increase a recording capacityof a storing medium (recording media). Therefore, in an optical disc asone of the recording media which have been most widespread at present,studies to further increase the recording capacity by increasing arecording density are vigorously being made.

For example, as a method of realizing the high recording density, amethod whereby a wavelength of a laser beam which is used torecord/reproduce an information signal is shortened and an NA (NumericalAperture) of an objective lens is increased, thereby reducing a beamspot diameter has been proposed.

For example, in an optical system of a CD (Compact Disc), asemiconductor laser which emits a laser beam having a wavelength of 780nm or 830 nm and an objective lens having an NA of 0.45 are provided. Inan optical system of a DVD (Digital Versatile Disc) which has beenwidespread in recent years, a semiconductor laser which emits a laserbeam having a wavelength of 660 nm and an objective lens having an NA of0.6 are provided. By providing such an optical system, the recordingcapacity of about 8 times of that of the CD can be realized in the DVD.

However, if the realization of such a high NA of the objective lens isprogressed, an aberration of light which is caused by an inclination ofthe disc increases, so that such a problem that a permission amount ofan inclination (tilt) of a disc surface to an optical axis of a pickupdecreases occurs. To solve such a problem, a method of decreasing athickness of substrate which transmits the laser beam has been proposed.For example, while the substrate having a thickness of 1.2 mm is used inthe CD, the substrate having a thickness of 0.6 mm is used in the DVD.

When considering the case of storing a video image of an HD (HighDefinition) or the like onto an optical disc in future, the recordingcapacity of the DVD is insufficient. Therefore, it is demanded torealize the shorter wavelength of the laser beam which is used torecord/reproduce the information signal, the higher NA of the objectivelens, and the thinner substrate.

Therefore, there has been proposed an optical disc of the nextgeneration in which a light transmitting layer having a thickness of 0.1mm is formed on an information signal portion formed on a substrate andan information signal is recorded/reproduced by irradiating a laser beamhaving a wavelength of 405 nm onto the information signal portion fromthe light transmitting layer side through an objective lens having an NAof 0.85. Since the optical disc of the next generation as mentionedabove has a structure in which the laser beam is inputted from the lighttransmitting layer side instead of the substrate side, the permissionamount of the tilt can be set to a sufficient large value in spite ofthe high NA of 0.85.

Upon manufacturing of the optical disc of the next generation, it isrequired to suppress a warp and an eccentricity more than those of theconventional optical disc. For this purpose, upon manufacturing of theoptical disc of the next generation, in order to guarantee mechanicalcharacteristics of a final product, it is important that the mechanicalcharacteristics of the transparent substrate just after the molding aremeasured at the earlier stage in the manufacturing step and they are fedback quickly.

Hitherto, as a method of measuring the mechanical characteristics suchas inclination, eccentricity, and the like of the optical disc, ameasuring method such as an optical stylus method or the like has beenproposed (for example, refer to JP-A-3-120640).

In a mechanical characteristics measuring apparatus using the opticalstylus method, it is necessary to provide a pickup according to a formatof the optical disc whose mechanical characteristics are measured, thatis, thicknesses of the substrate and the light transmitting layer and avalue of the track pitch. This is because in the case of measuring themechanical characteristics of the optical disc by using the opticalstylus method, it is necessary to allow the light converged by thepickup to trace grooves.

A method of measuring the mechanical characteristics of the discsubstrate which is used for the optical disc of the next generation byusing the mechanical characteristics measuring apparatus using theoptical stylus method has been proposed. According to such a method, atleast a reflective film and a light transmitting layer having athickness of 0.1 mm are formed on the disc substrate and, by irradiatingthe laser beam from the light transmitting layer side, the mechanicalcharacteristics of the disc substrate are measured. By forming at leastthe reflective film and the light transmitting layer having a thicknessof 0.1 mm onto the disc substrate as mentioned above, the mechanicalcharacteristics of the disc substrate can be measured.

However, in order to measure the mechanical characteristics of the discsubstrate as mentioned above, the light transmitting layer of 0.1 mm hasto be formed and the mechanical characteristics of the disc substratecannot be measured in the state of the transparent substrate just afterthe molding. Consequently, a feedback speed upon manufacturing becomesslow, so that the productivity of the optical disc is deteriorated.

Therefore, a method whereby a pickup which can converge the laser beamonto each groove formed on the disc substrate in the state where thelight transmitting layer of 0.1 mm is not formed is designed andequipped for the mechanical characteristics measuring apparatus has beenproposed. However, if such a pickup is designed and equipped for themechanical characteristics measuring apparatus only for the purpose ofmeasuring the mechanical characteristics of the transparent substrate,expenses for manufacturing facilities are raised.

Therefore, a method of measuring the mechanical characteristics of theoptical disc of the next generation by using the mechanicalcharacteristics measuring apparatus of the optical disc using theoptical stylus method which has conventionally been widespread has beenproposed. The mechanical characteristics measuring apparatus is used tomeasure an eccentricity amount of the disc substrate having a thicknessof 1.2 mm and is constructed by a semiconductor laser which emits thelaser having a wavelength of 680 nm and a pickup having an objectivelens of an NA of 0.55. In the foregoing optical disc of the nextgeneration, since the substrate having a thickness of about 1.1 mm isused, by converging the laser beam through the disc substrate, a surfaceoscillation amount, the inclination of the disc, and the like can bealso measured by such a conventional mechanical characteristicsapparatus.

However, in the format of the optical disc of the next generationmentioned above, since the track pitch is equal to or less than 0.6 μm,a tracking error signal of a sufficient level cannot be obtained by theoptical system equipped for the conventional mechanical characteristicsmeasuring apparatus. In other words, the eccentricity amount cannot bemeasured by the conventional mechanical characteristics measuringapparatus.

DISCLOSURE OF INVENTION

It is, therefore, an object of the invention to provide a disc substrateand an optical disc whose eccentricity amount can be easily measured inthe state of the transparent substrate just after molding in the opticaldisc in which an interval between grooves of a data area is equal to orless than 0.6 μm.

The present inventors have eagerly examined to solve the foregoingproblems of the conventional techniques. An outline of the examinationwill be described hereinbelow.

According to the knowledge of the present inventors, the reason why theeccentricity amount of the disc substrate whose track pitch is equal toor less than 0.6 μm cannot be measured by the conventional mechanicalcharacteristics measuring apparatus is because the tracking error signalof a sufficient level cannot be obtained from the disc substrate of sucha format.

To solve the foregoing problems, the present inventors have vigorouslyexamined with respect to a method whereby in the disc substrate whosetrack pitch is equal to or less than 0.6 μm, the tracking error signalof a sufficient level can be obtained by the conventional mechanicalcharacteristics measuring apparatus. Thus, the present inventors havefound a method whereby an eccentricity measuring area to measure theeccentricity is provided and an interval between the grooves is widenedonly in the eccentricity area.

However, the present inventors have further examined with respect tosuch a method, so that they have found that the methd has the followingproblems.

Generally, in the case of forming a thin groove in which a grooveinterval in a data area is equal to or less than 0.6 μm, a wavelength ofan exposing laser upon mastering has to be also shortened incorrespondence to such a thin groove and a laser of, for example, awavelength of 266 nm is used.

However, they have found that there is such a problem that in the caseof using such a laser of the short wavelength, if the track pitch iswidened in the foregoing eccentricity measuring area, since a groovewidth to the track pitch is too narrow, the sufficient push-pull signalcannot be obtained and, further, a signal waveform is also distorted.

By further making the examination as mentioned above, the inventors havefound a method whereby an eccentricity measuring area constructed by agroove area in which spiral grooves have been formed and a planer mirrorarea adjacent to the groove area is provided for the disc substrate.

The invention has been made on the basis of the above examination.

Therefore, to solve the above problems, according to the first inventionof the present invention, there is provided a disc substrate having aneccentricity measuring area in which a groove area formed with spiralgrooves and a planer mirror area are spatially alternately arranged.

According to the second invention of the present invention, there isprovided an optical disc comprising:

a disc substrate having an eccentricity measuring area in which a groovearea formed with spiral grooves and a planer mirror area are spatiallyalternately arranged;

an information signal portion formed on one principal plane of the discsubstrate; and

a protective layer for protecting the information signal portion.

As mentioned above, according to the invention, since the disc substratehas the eccentricity measuring area in which the groove area formed withthe spiral grooves and the planer mirror area are spatially alternatelyarranged, the conventional mechanical characteristics measuringapparatus can discriminate the groove area formed with the spiralgrooves as if the groove area were a single groove.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view showing a structure of an optical discaccording to an embodiment of the invention.

FIG. 2 is a cross sectional view showing a construction of a substrateaccording to the embodiment of the invention.

FIG. 3 is a cross sectional view showing a construction of a sheetaccording to the embodiment of the invention.

FIG. 4 is a perspective view of a disc substrate according to theembodiment of the invention.

FIG. 5 is a plan view of an eccentricity measuring area provided for thedisc substrate according to the embodiment of the invention.

FIG. 6 is a schematic diagram showing a waveform of a push-pull signalat a joint.

FIG. 7 is a schematic diagram showing a waveform of a push-pull signalwhich is caused when a pickup is moved to an adjacent groove area in theeccentricity measuring area.

FIG. 8 is a cross sectional view showing an image upon reproduction ofdata of the optical disc according to the embodiment of the invention.

FIG. 9 is a cross sectional view showing an image at the time ofmeasurement of mechanical characteristics of the disc substrateaccording to the embodiment of the invention.

FIG. 10 is a plan view of an eccentricity measuring area provided for adisc substrate according to a modification of the embodiment of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described hereinbelow withreference to the drawings. The same or corresponding portions in alldiagrams of the following embodiments are designated by the samereference numerals.

FIG. 1 shows an example of a construction of an optical disc accordingto an embodiment of the invention. FIG. 2 shows an example of aconstruction of a substrate according to the embodiment of theinvention. FIG. 3 shows an example of a construction of a sheetaccording to the embodiment of the invention.

As shown in FIG. 1, the optical disc according to the embodiment of theinvention is mainly constructed by: an annular ring-shaped substrate 1having a center hole 1 b in a center portion; and a planer annularring-shaped light transmitting layer 2 having a through-hole 2 c in thecenter portion. The optical disc according to the embodiment isconstructed in such a manner that an information signal is recordedand/or reproduced by irradiating a laser beam onto the substrate 1 fromthe side where the thin light transmitting layer 2 is formed. The lighttransmitting layer 2 is formed by adhering a sheet 4 shown in FIG. 3onto one principal plane of the substrate 1 shown in FIG. 2 on the sidewhere an information signal portion 1 c has been formed.

As shown in FIG. 1, a clamp area 3 to attach the optical disc to aspindle motor is provided near the center hole 1 b of the optical disc.An inner rim diameter of the clamp area 3 is selected from a range of 22to 24 mm and, for example, 23 mm is selected. An outer rim diameter ofthe clamp area 3 is selected from a range of 32 to 34 mm and, forexample, 33 mm is selected.

As shown in FIG. 2, the substrate 1 is constructed by: a disc substrate1 a in which the center hole 1 b is formed in the center portion andlands and grooves are formed on one principal plane; and the informationsignal portion 1 c formed on one principal plane of the disc substrate 1a. A data area and an eccentricity measuring area are provided for thearea where the lands and grooves have been formed. In the embodiment ofthe invention, on one principal plane of the disc substrate 1 a, theportion near the incident light is called a groove and the portionformed between the grooves is called a land.

FIG. 4 is a perspective view of the disc substrate 1 a according to theembodiment of the invention. As shown in FIG. 4, a non-data area 11 toattach the disc substrate 1 a to the spindle motor, a data area 12 toform the information signal portion 1 c, and a non-data area 13 havingan eccentricity measuring area 14 to measure the eccentricity of thedisc substrate 1 a are sequentially provided on the disc substrate 1 afrom the inner rim side toward the outer rim side. Although an examplein which the eccentricity measuring area 14 is formed in the non-dataarea 13 provided on the outer rim side is shown here, the eccentricitymeasuring area can be also formed in the non-data area 11 provided onthe inner rim side.

A thickness of disc substrate 1 a is selected from a range of 0.6 to 1.2mm and, for example, 1.1 mm is selected. A diameter (outer diameter) ofthe disc substrate 1 a is equal to, for example, 120 mm. An openingdiameter (inner diameter) of the center hole 1 b is equal to, forexample, 15 mm. In the data area 12, data is recorded either on thegroove or on the land, or on both of them. A case where a system ofrecording the data onto the groove is selected is shown hereinbelow. Adistance between the grooves formed in the data area 12 (track pitch) isset to, for example, 0.32 μm. A width of groove formed in the data area12 is selected in consideration of signal characteristics and, forexample, 0.22 μm (half value width) is selected.

The disc substrate 1 a is made of a material which can transmit thelaser beam which is used to measure at least the mechanicalcharacteristics of the disc substrate 1 a. As a material constructingthe disc substrate 1 a, a resin with low water absorption performancesuch as polycarbonate (PC), cycloolefin polymer (for example, ZEONEX(registered trademark)), or the like is used.

The information signal portion 1 c is constructed by a reflective film,a film made of a magnetooptic material, a film made of a phase changematerial, an organic pigment film, or the like. Specifically speaking,when the optical disc as a final product is an optical disc of the ROM(Read Only Memory) type, the information signal portion 1 c isconstructed by a single layer film or a laminate film each having atleast a reflective layer made of, for example, Al, an Al alloy, an Agalloy, or the like. When the optical disc as a final product is anoptical disc of a rewritable type, the information signal portion 1 c isconstructed by a single layer film or a laminate film each having atleast either a film made of a magnetooptic material such as TbFeCoalloy, TbFeCoSi alloy, TbFeCoCr alloy, or the like or a film made of aphase change material such as GeSbTe alloy, GeInSbTe alloy, AgInSbTealloy, or the like. When the optical disc as a final product is anoptical disc of a WORM (Write Once Read Many) type, the informationsignal portion 1 c is constructed by a single layer film or a laminatefilm each having at least either a film made of a phase change materialsuch as a GeTe material or the like or a film made of an organic pigmentmaterial such as cyanine dye, phthalocyanine dye, or the like.

The eccentricity measuring area 14 is an area for measuring aneccentricity amount of the optical disc, specifically speaking, an areafor measuring the eccentricity amount of the optical disc by using theconventional mechanical characteristics measuring apparatus of theoptical disc. In the embodiment of the invention, for example, there isshown a case where the conventional mechanical characteristics measuringapparatus is a mechanical characteristics measuring apparatus formeasuring the mechanical characteristics of the optical disc (forexample, compact disc) in which a thickness of substrate is equal to 1.2mm, specifically speaking, it is a mechanical characteristics measuringapparatus having an optical system including a semiconductor laser whichemits a laser beam of a wavelength of 680 nm and an objective lens of anNA of 0.55.

FIG. 5 shows a plan view of the eccentricity measuring area 14 formed onone principal plane of the disc substrate 1 a. As shown in FIG. 5, theeccentricity measuring area 14 is constructed in such a manner that agroove area in which spiral grooves have been formed and a planer mirrorarea are spatially alternately arranged. A width of eccentricitymeasuring area 14 is selected so as to have a value which is equal to orlarger than the maximum value of the amount of eccentricity which iscaused in the manufacturing step of the disc substrate 1 a. Since themaximum value of the amount of eccentricity which is caused in theconventional manufacturing step of the disc substrate 1 a is equal toabout 30 μm, at least 30 μm or more is necessary as a width ofeccentricity measuring area 14. Its upper limit is not restricted interms of the eccentricity measurement. However, if the width ofeccentricity measuring area 14 is too wide, a width of data area 12decreases. It is, therefore, preferable to set the width of eccentricitymeasuring area 14 to 3 mm or less. From the above viewpoint, theeccentricity measuring area 14 is selected from a range of 30 μm to 3 mmand, for example, 100 μm is selected.

The spiral grooves are formed in the groove area around the center hole1 b as a center. By forming the grooves as mentioned above, theconventional mechanical characteristics measuring apparatus can obtain atracking error signal (push-pull signal) of a sufficient level. That is,the conventional mechanical characteristics measuring apparatus canexecute the proper tracking operation.

A repetition interval d₃ of the groove area or the mirror area isselected in accordance with the optical system of the mechanicalcharacteristics measuring apparatus. That is, the repetition interval d₃is selected so that the optical system of the mechanical characteristicsmeasuring apparatus can track the groove area as if the groove area werea single groove. In the case of using the conventional mechanicalcharacteristics measuring apparatus mentioned above, the repetitioninterval d₃ of the groove area or the mirror area is selected from arange of 0.7 to 2.5 μm and, for example, 1.6 μm is selected. When therepetition interval d₃ is equal to or more than 0.7 μm, in themechanical characteristics measuring apparatus having the optical systemas mentioned above, the tracking error signal (push-pull signal) of thesufficient level can be obtained. That is, the stable tracking operationcan be executed. If the repetition interval d₃ is equal to or less than2.5 μm, in the mechanical characteristics measuring apparatus having theoptical system as mentioned above, the tracking error signal (push-pullsignal) with small distortion can be obtained.

A width of groove area is selected in accordance with the optical systemof the mechanical characteristics apparatus for measuring the mechanicalcharacteristics of the optical disc. That is, it is selected so that theoptical system of the mechanical characteristics measuring apparatus cantrack the groove area as if the groove area were a single groove.

When the mechanical characteristics of the optical disc are measured byusing the conventional mechanical characteristics measuring apparatus asmentioned above, if the push-pull signal of the sufficient level withouta distortion can be obtained, an arbitrary value can be selected as awidth of groove area. Generally, by selecting the width of groove areato a value within a range of 0.2 to 0.8 time of the repetition intervald₃ of the groove area mentioned above, the above characteristics can besatisfied. Particularly, by selecting the width of groove area to avalue which is equal to almost the half of the repetition interval d₃ ofthe groove area, the push-pull signal of the maximum amplitude with thesuppressed distortion can be obtained. For example, if the repetitioninterval d₃ of the groove area is selected so as to be 1.6 μm, the widthof groove area is selected so as to be, for example, 0.8 μm.

The mirror area formed adjacently to the groove area is a plane area onwhich no grooves are formed. A width of mirror area is selected inaccordance with the optical system of the mechanical characteristicsmeasuring apparatus for measuring the mechanical characteristics of theoptical disc. That is, it is selected so that the optical system of themechanical characteristics measuring apparatus can track the groove areaas if the groove area were a single groove.

When the mechanical characteristics of the optical disc are measured byusing the conventional mechanical characteristics measuring apparatus asmentioned above, if the push-pull signal of the sufficient level withouta distortion can be obtained, an arbitrary value can be selected as awidth of mirror area. Generally, by selecting the width of mirror areato a value within a range of 0.2 to 0.8 time of the repetition intervald₃ of the groove area mentioned above, the above characteristics can besatisfied. Particularly, by selecting the width of mirror area to avalue which is equal to almost the half of the repetition interval d₃ ofthe groove area, the push-pull signal of the maximum amplitude with thesuppressed distortion can be obtained. For example, if the repetitioninterval d₃ of the groove area is selected so as to be 1.6 μm, the widthof mirror area is selected so as to be, for example, 0.8 μm.

It is generally unpreferable to spirally and intermittently form thegrooves in the groove area. If the grooves are spirally andintermittently formed, the center of reproduction light is deviated at ajoint from the center of the groove. Therefore, the push-pull signalfluctuates and the stable tracking operation cannot be executed. Thejoint indicates one end and the other end of the groove spirally formedin the groove area.

According to the optical disc and the disc substrate 1 a of theembodiment, by properly selecting an interval d₁ between the grooves inthe groove area, the fluctuation of the push-pull signal at the joint isreduced, thereby realizing the stable tracking operation.

The interval d₁ between the grooves in the groove area is determined inconsideration of the optical system of the mechanical characteristicsmeasuring apparatus which is used to measure the eccentricity of thedisc substrate 1 a and the fluctuation of the push-pull signal at thejoint. When considering them, the interval d₁ between the grooves in thegroove area is equal to or less than a diffraction limit of the opticalsystem of the mechanical characteristics measuring apparatus formeasuring the eccentricity of the disc substrate 1 a and is selected soas to have a value of 0.01 to 0.25 time, preferably, 0.01 to 0.15 timeof the repetition interval d₃ of the groove area or the mirror area.

For example, in the case of using the conventional mechanicalcharacteristics measuring apparatus mentioned above, the interval d₁between the grooves in the groove area is equal to or less than 0.6 μmand is selected so as to have a value of 0.01 to 0.25 time, preferably,0.01 to 0.15 time of the repetition interval between the groove area orthe mirror area.

The diffraction limit of the optical system of the conventionalmechanical characteristics measuring apparatus mentioned abovecorresponds to 0.6 μm as a spatial period. Therefore, by setting theinterval between the grooves in the groove area to 0.6 μm or less, inthe conventional mechanical characteristics apparatus, each groove inthe groove area is not identified but a plurality of grooves formed inthe groove area are identified as if the grooves were a single groove.

The interval d₁ between the grooves in the case of considering thefluctuation of the push-pull signal at the joint will now be describedwith reference to FIGS. 6 and 7.

FIG. 6 shows a waveform of the push-pull signal at the joint. As shownin FIG. 6, a fluctuation occurs in the push-pull signal at the joint. Anamount which is offset at this time is called an offset amount Bhereinbelow.

FIG. 7 shows a waveform of the push-pull signal which is caused when thepickup is moved to the adjacent groove area. As shown in FIG. 7, thewaveform of the push-pull signal which is caused when the pickup ismoved to the adjacent groove area has an S-character shape. An amplitudeat this time is called an amplitude (A) hereinbelow.

An upper limit value of the interval d₁ between the grooves in thegroove area is selected so that the offset amount B is equal to or lessthan the amplitude (A). The offset amount B and the amplitude (A) areequal when a detrack amount at the joint is equal to almost 0.25 time ofthe width of groove area. Therefore, to prevent the tracking positionfrom being detracked at the joint, it is necessary to select the detrackamount at the joint, that is, the interval d₁ between the grooves in thegroove area so as to be equal to or less than 0.25 time of the intervalbetween the groove areas.

To realize the more stable tracking operation without being influencedby a disturbance, it is preferable to select the detrack amount at thejoint so that the offset amount B is equal to a smaller value, forexample, the offset amount B is equal to or less than 0.8 time of theamplitude (A). The offset amount B is set to 0.8 time of the amplitude(A) when the detrack amount at the joint, that is, the interval d₁between the grooves formed in the groove area is selected so as to beabout 0.15 time of the interval between the groove areas.

A lower limit value of the interval d₁ between the grooves formed in thegroove area is not particularly limited. When considering theproductivity, however, it is preferable that the lower limit value isequal to or more than 0.01 time of the repetition interval d₃ of thegroove area. By selecting the interval d₁ between the grooves asmentioned above, the situation in which it takes time to execute thecutting operation of a mother disc and the productivity is deterioratedcan be avoided.

As shown in FIG. 3, the sheet 4 used to form the light transmittinglayer 2 according to the embodiment is constructed by: a lighttransmitting sheet 2 a; and an adhesive layer 2 b made of a PSA(Pressure Sensitive Adhesion) adhered to one surface of the lighttransmitting sheet 2 a. In a manner similar to the substrate 1, thesheet 4 has a structure punched in a planer annular ring shape and thethrough-hole 2 c is formed in the center portion. A diameter (outerdiameter) of the sheet 4 is selected so as to have a value which isalmost equal to or less than the outer diameter of the substrate land,for example, 120 mm is set. A diameter (inner diameter) of thethrough-hole 2 c is selected from a range from the opening diameter ofthe center hole 1 b or more to the innermost rim diameter (for example,23 mm diameter) of the clamp area 3 and it is set to, 23 mm. A thicknessof sheet 4 is equal to, for example, 100 μm.

Such a light transmitting sheet 2 a of the sheet 4 is made of, forexample, a thermoplastic resin with light transmittance which is used atleast for recording and/or reproduction and satisfies the opticalcharacteristics at which the laser beam can be transmitted. A materialof the thermoplastic resin is selected from materials whose physicalproperty values such as heat resisting dimensional stability,coefficient of thermal expansion, coefficient of hydroscopic expansion,and the like are close to those of the disc substrate 1 a. Specificallyspeaking, it is selected from polycarbonate (PC), a methacrylic resinsuch as polymethyl methacrylate, and the like. A thickness of lighttransmitting sheet 2 a is selected preferably from a range of 60 to 100μm, more preferably, 70 to 100 μm. In the embodiment, the thickness oflight transmitting sheet 2 a is selected so as to be, for example, 70 μmin consideration of the structure in which the light transmitting sheet2 a is adhered onto one principal plane of the substrate 1 through theadhesive layer 2 b made of the PSA (Pressure Sensitive Adhesion). Thethickness of light transmitting sheet 2 a is determined in considerationof the wavelength of the laser beam which is used to record and/orreproduce the information signal and the desired film thickness of thelight transmitting layer 2.

The PSA constructing the adhesive layer 2 b is, for example, themethacrylic resin or the like. A thickness of adhesive layer 2 b isequal to, for example, 30 μm. The thickness of adhesive layer 2 b andthe material which is used as a pressure sensitive adhesion aredetermined in consideration of the desired film thickness of the lighttransmitting layer 2 and the wavelength of the laser beam which is usedto record and/or reproduce the information signal.

FIG. 8 is a cross sectional view showing an image upon reproduction ofthe optical disc according to the embodiment of the invention. As shownin FIG. 8, in the optical disc according to the embodiment of theinvention, the information signal is recorded and/or reproduced byirradiating the laser beam to the information signal portion 1 c of thesubstrate 1 from the side where the thin light transmitting layer 2 hasbeen formed.

FIG. 9 is a cross sectional view showing an image at the time of themeasurement of the mechanical characteristics of the disc substrate 1 aaccording to the embodiment of the invention. As shown in FIG. 9, in thedisc substrate 1 a according to the embodiment of the invention, themechanical characteristics of the disc substrate 1 a are measured byirradiating the laser beam to the surface on the side opposite to oneprincipal plane of the side where concave and convex portions have beenformed.

According to the embodiment of the invention, the following effects canbe obtained.

The data area 12 formed with the spiral grooves and the eccentricitymeasuring area on which the groove area formed with the spiral groovesand the planer mirror area have alternately been arranged are providedon the disc substrate 1 a. The interval d₁ between the grooves in thegroove area is equal to or less than the diffraction limit of theoptical system of the conventional mechanical characteristics measuringapparatus which is used to measure the eccentricity of the discsubstrate and is selected so as to have a value of 0.01 to 0.25 time,preferably, 0.01 to 0.15 time of the repetition interval d₃ of thegroove area or the mirror area. Therefore, the conventional mechanicalcharacteristics measuring apparatus can stably track the groove area asif the apparatus tracked a single groove. Consequently, the eccentricityamount of the optical disc of the narrow track pitch having the spiralgrooves can be measured by using the conventional mechanicalcharacteristics measuring apparatus.

Since the eccentricity amount of the disc substrate 1 a can be measuredin the state where the light transmitting layer 2 is not formed, theoptical disc can be manufactured by the efficient producing system.

Although the embodiment of the invention has specifically been describedabove, the invention is not limited to the foregoing embodiment butvarious modifications based on the technical idea of the invention arepossible.

For instance, the numerical values mentioned in the foregoing embodimenthave merely been shown as an example and other different numericalvalues can be also used as necessary.

Although the example in which the distance d₁ between the grooves formedin the eccentricity measuring area 14 and the distance between thegrooves formed in the data area 12 are different has been shown in theforegoing embodiment, the distance d₁ between the grooves formed in theeccentricity measuring area 14 and the distance between the groovesformed in the data area 12 can be made coincident.

Although the case where, in the groove area formed with the spiralgrooves, an end position of the intermittent groove (position of one endof the groove on the outer rim side) and a start position of theintermittent groove (position of one end of the groove on the inner rimside) are located in the same direction from the center of the discsubstrate 1 a has been shown as an example, the end position of theintermittent groove and the start position of the intermittent grooveare not limited to such an example.

For example, as shown in FIG. 10, the direction starting from the centerof the disc substrate 1 a toward the start position of the intermittentgroove and the direction starting from the center of the disc substrate1 a toward the end position of the intermittent groove can be madedifferent by 180°. In this case, although the width of groove areadiffers depending on the position, there is such an advantage that thedeviation amount of the center of the groove area at the joint isreduced to the half of the interval d₁ between the grooves in the groovearea.

As described above, according to the invention, since the disc substratehas the eccentricity measuring area on which the groove area formed withthe spiral grooves and the planer mirror area have spatially alternatelybeen arranged, the conventional mechanical characteristics measuringapparatus can discriminate the groove area formed with the spiralgrooves as if the groove area were a single groove. Therefore, theeccentricity amount of the disc substrate can be easily measured in thestate just after the molding.

1. A disc substrate having an eccentricity measuring area in which agroove area formed with spiral grooves and a planer mirror area arespatially alternately arranged.
 2. A disc substrate according to claim1, wherein an interval between the grooves in said groove area isselected in accordance with an optical system of a mechanicalcharacteristics measuring apparatus which is used to measure aneccentricity amount and a fluctuation of a push-pull signal at one endand the other end of said groove formed spirally in said groove area. 3.A disc substrate according to claim 2, wherein a width of said groovearea and a width of said mirror area are selected in accordance with theoptical system of said mechanical characteristics measuring apparatuswhich is used to measure the eccentricity amount.
 4. A disc substrateaccording to claim 2, wherein an interval between said grooves isselected so as to have a value in a range from 0.01 time or more to 0.25time or less of a repetition interval of said groove area or said mirrorarea.
 5. A disc substrate according to claim 2, wherein an intervalbetween said grooves is selected so as to have a value in a range from0.01 time or more to 0.15 time or less of a repetition interval of saidgroove area or said mirror area.
 6. A disc substrate according to claim4, wherein the repetition interval of said groove area or said mirrorarea is set to a value in a range from 0.7 μm or more to 2.5 μm or less.7. A disc substrate according to claim 4, wherein a width of said groovearea is selected so as to have a value in a range from 0.2 time or moreto 0.8 time or less of the repetition interval of said groove area orsaid mirror area.
 8. A disc substrate according to claim 4, wherein awidth of said groove area is equal to almost the half of the repetitioninterval of said groove area or said mirror area.
 9. A disc substrateaccording to claim 4, wherein a width of said eccentricity measuringarea is selected so as to have a value in a range from 30 μm or more to3 mm or less.
 10. A disc substrate according to claim 1, wherein a clamparea to attach an optical disc to a spindle motor is set near a centerhole of said disc substrate, an inner rim diameter of said clamp area isselected from a range of 22 to 24 mm, and an outer rim diameter of saidclamp area is selected from a range of 32 to 34 mm.
 11. A disc substrateaccording to claim 1, wherein a non-data area to attach the discsubstrate to a spindle motor, a data area to form an information signalportion, and a non-data area having the eccentricity measuring area tomeasure eccentricity of the disc substrate are sequentially provided.12. A disc substrate according to claim 1, wherein a thickness of saiddisc substrate is selected from a range of 0.6 to 1.2 mm, a diameter(outer diameter) of said disc substrate is equal to 80 to 120 mm, and anopening diameter (inner diameter) of a center hole is equal to about 15mm.
 13. A disc substrate according to claim 1, wherein in a system forrecording onto the grooves, a distance (track pitch) between the groovesformed in a data area is equal to about 0.32 μm and a width of eachgroove formed in the data area is equal to about 0.22 μm (half valuewidth).
 14. An optical disc comprising: a disc substrate having aneccentricity measuring area in which a groove area formed with spiralgrooves and a planer mirror area are spatially alternately arranged; aninformation signal portion formed on one principal plane of said discsubstrate; and a protective layer for protecting said information signalportion.
 15. An optical disc according to claim 14, wherein saidprotective layer has light transmittance and recording and/orreproduction of an information signal are/is executed by irradiating alaser beam from the side where said protective layer is provided.
 16. Anoptical disc according to claim 14, wherein an interval between thegrooves in said groove area is selected in accordance with an opticalsystem of a mechanical characteristics measuring apparatus which is usedto measure an eccentricity amount and a fluctuation of a push-pullsignal at one end and the other end of said groove formed spirally insaid groove area.
 17. An optical disc according to claim 16, wherein awidth of said groove area and a width of said mirror area are selectedin accordance with the optical system of said mechanical characteristicsmeasuring apparatus which is used to measure the eccentricity amount.18. An optical disc according to claim 16, wherein an interval betweensaid grooves is selected so as to have a value in a range from 0.01 timeor more to 0.25 time or less of a repetition interval of said groovearea or said mirror area.
 19. An optical disc according to claim 16,wherein an interval between said grooves is selected so a to have avalue in a range from 0.01 time or more to 0.15 time or less of arepetition interval of said groove area or said mirror area.
 20. Anoptical disc according to claim 18, wherein the repetition interval ofsaid groove area or said mirror area is set to a value in a range from0.7 μm or more to 2.5 μm or less.
 21. An optical disc according to claim18, wherein a width of said groove area is selected so as to have avalue in a range from 0.2 time or more to 0.8 time or less of therepetition interval of said groove area or said mirror area.
 22. Anoptical disc according to claim 18, wherein a width of said groove areais equal to almost the half of the repetition interval of said groovearea or said mirror area.
 23. An optical disc according to claim 18,wherein a width of said eccentricity measuring area is set to a value ina range from 30 μm or more to 3 mm or less.
 24. An optical discaccording to claim 14, wherein said protective layer is made of a lighttransmitting layer and formed by adhering a sheet onto one principalplane of the substrate on the side where said information signal portionhas been formed.
 25. An optical disc according to claim 14, wherein aclamp area to attach an optical disc to a spindle motor is set near acenter hole of said disc substrate, an inner rim diameter of said clamparea is selected from a range of 22 to 24 mm, and an outer rim diameterof said clamp area is selected from a range of 32 to 34 mm.
 26. Anoptical disc according to claim 14, wherein a non-data area to attachthe disc substrate to a spindle motor, a data area to form theinformation signal portion, and a non-data area having an eccentricitymeasuring area to measure eccentricity of the disc substrate aresequentially provided.
 27. An optical disc according to claim 14,wherein a thickness of said disc substrate is selected from a range of0.6 to 1.2 mm, a diameter (outer diameter) of said disc substrate isequal to 80 to 120 mm, and an opening diameter (inner diameter) of acenter hole is equal to about 15 mm.
 28. An optical disc according toclaim 14, wherein in a system for recording onto the grooves, a distance(track pitch) between the grooves formed in a data area is equal toabout 0.32 μm and a width of each groove formed in the data area isequal to about 0.22 μm (half value width).
 29. An optical disc accordingto claim 14, wherein the sheet which is used to form said lighttransmitting layer comprises a light transmitting sheet and a PSA(Pressure Sensitive Adhesion) adhered to one surface of said lighttransmitting sheet.